1. Field of the Invention
The present invention relates generally to metal production and, in certain embodiments, relates to methods of producing metallic tubular bars having high strength and improved formability, and metal bars, particularly vehicular stabilizer bars, made by the same.
2. Description of the Related Art
Metal tubular bars possess a number of advantages over solid bars when incorporated into engineered structures. For example, tubular bars are lighter than corresponding solid bars, of benefit to applications in which weight is a primary consideration. Automobile stabilizer bars are one example of the use of metal tubular bars. These bars are attached to an auto frame between the tires and function to counteract roll which occurs during turning, ensuring more even wear on tires and greater steering control when turning. Unfortunately, owing to their reduced load-bearing cross-section, considering materials with identical mechanical properties, metal tubular bars also fail at lower loads than solid tubular bars of equal diameter
The fabrication of metal tubular bar components has focused on maintaining their static strength and fatigue resistance while improving formability, the ability to shape a part to a desired configuration without damage. There are two general methods which have been developed to form and strengthen metal tubular bars, particularly steel tubular bars. In the first method, a tubular bar is hot or cold bent and then heat treated to strengthen the bar. In one implementation of this method, the tubular bar is initially hot or cold formed to shape when the metal is relatively soft, allowing the metal to be easily manipulated. Subsequently, the formed bar is heated and quenched to increase the strength of the steel. Following the quench, the hardened bar is mildly tempered to soften the steel while improving the toughness of the bar. Finally, the exterior surface of the tubular bar may be shot-peened to introduce a state of compressive stresses onto the surface which inhibits the growth of flaws under fatigue loadings which can lead to failure.
Often, though, the thermal stresses generated in the tubular bar during the quench distort the shape of the tubular bar, requiring time consuming and costly re-forming operations to return the tubular bar to its desired shape.
In the second method of fabricating metal tubular bar components, the tubular bar is formed after a strengthening operation. One embodiment of this method utilizes cold drawing to strengthen the tube, while another embodiment utilizes a heat treated tube for this purpose. Following the heat treatment, the tubular bar is bent to shape and then tempered to age-harden the metal. Lastly, shot-peening may be performed on the external and, in some cases, internal surfaces of the tubular bar.
The above methods require considerable time and effort to produce a high strength tubular bar with the desired properties. Accordingly, what is needed are bars and methods of manufacture offering improvements over these prior methods.